diff --git a/changelog.d/9484.bugfix b/changelog.d/9484.bugfix
new file mode 100644
index 000000000000..b55f7d20ae96
--- /dev/null
+++ b/changelog.d/9484.bugfix
@@ -0,0 +1 @@
+Add experimental config feature for timed `/sync` cache, fixes #3880.
\ No newline at end of file
diff --git a/synapse/config/experimental.py b/synapse/config/experimental.py
index b1c1c51e4dcc..54bb7b3b9530 100644
--- a/synapse/config/experimental.py
+++ b/synapse/config/experimental.py
@@ -27,3 +27,8 @@ def read_config(self, config: JsonDict, **kwargs):
 
         # MSC2858 (multiple SSO identity providers)
         self.msc2858_enabled = experimental.get("msc2858_enabled", False)  # type: bool
+
+        # Timeout value (in milliseconds) on sync Response Cache
+        self.sync_cache_timeout_ms = experimental.get(
+            "sync_cache_timeout_ms", 0
+        )  # type: int
diff --git a/synapse/handlers/sync.py b/synapse/handlers/sync.py
index ce644e01ad0d..04ac48b809b3 100644
--- a/synapse/handlers/sync.py
+++ b/synapse/handlers/sync.py
@@ -244,7 +244,7 @@ def __init__(self, hs: "HomeServer"):
         self.event_sources = hs.get_event_sources()
         self.clock = hs.get_clock()
         self.response_cache = ResponseCache(
-            hs, "sync"
+            hs, "sync", timeout_ms=hs.config.experimental.sync_cache_timeout_ms
         )  # type: ResponseCache[Tuple[Any, ...]]
         self.state = hs.get_state_handler()
         self.auth = hs.get_auth()